Composition for producing a conductive composite material containing a polyaniline, and resulting composite material

ABSTRACT

The invention concerns compositions for manufacturing composite materials containing a polyaniline. 
     These compositions are formed by a solution in a solvent such as m-cresol of the following constituents: 
     a) a conductive polyaniline protonated by means of a protonation agent able to promote the dissolution of the polyaniline in the solvent, for example phenylphosphonic acid, 
     b) an insulating polymer chosen for example from amongst the cellulosic polymers and polyvinyl chlorides such as cellulose acetate, and 
     c) an insulating plasticiser such as a mixture of dimethyl phthalate, diethyl phthalate and triphenyl phosphate. 
     By pouring this solution and evaporating the solvent, it is possible to obtain a flexible film of conductive composite material having good electrical and mechanical properties.

DESCRIPTION

1. Technical Field

The object of the present invention is the manufacture of electricallyconductive composite materials containing a polyaniline.

It concerns in particular the manufacture of highly transparentconductive films, having good mechanical properties, which comprise aninsulating polymer host matrix in which there is distributed aconductive polyaniline conferring electrical conductivity on the whole.

Films of this type can be used in particular in electrostatic shieldingor de-icing windows.

2. State of the Prior Art

In order to obtain electrical conductivity with composite materials ofthis type, it is necessary for the conductive polymer which constitutesthe conductive phase to form a continuous lattice in the material. Thiscan be obtained only as from a certain threshold referred to as the“percolation threshold”, which can be defined as the conductive phaseminimum fraction by volume which makes the material conductive on amacroscopic scale. This percolation threshold can be determined from thefollowing formula:

σ(f)=c(f−f_(c))^(t)

in which:

σ represents the conductivity,

c is a constant,

t is the critical exponent,

f represents the fraction by volume of the conductive phase,

f_(c) is the fraction by volume of the conductive phase at thepercolation threshold.

The publication by M. A. Knackstedt and A. P. Roberts in Macromolecules,29, 1996, pp 1369-1371, gives explanations on the percolation threshold.

This threshold depends strongly on the morphology of the conductivephase. Thus, when the conductive phase consists of carbon black ormetals, the percolation threshold is generally very high and very oftengreater than 0.5. However, composite materials have recently beenproduced whose conductive phase is formed by carbon black, which have avery much lower percolation threshold (0.4% by weight), as described byGubbll et al in Macromolecules, 28, 1995, pp 1559-1566.

In the case of composite materials where the conductive phase consistsof a conductive polymer, lower percolation thresholds can be expectedusing techniques of manufacturing from a solution or techniques ofmanufacturing by hot compression of a mixture of polymers in the solidstate.

The document U.S. Pat. No. 5,232,631 describes the manufacture ofcomposite materials from a solution of insulating polymer forming thehost matrix and a conductive polyaniline in a solvent. In this case, thepolyaniline is first of all caused to react with a suitable protonationagent which enables it to be made soluble in a suitable organic solvent.The solution is next used to form a film by pouring and evaporating thesolvent. With these techniques very low percolation thresholds and highconductivities can be achieved.

The document EP-A-0 643 397 describes the manufacture of conductivecomposite materials also comprising an insulating polymer host matrix inwhich there is distributed a conductive polymer consisting of apolyaniline, which is obtained by hot compression moulding of a mixtureof conductive polymer and the insulating polymer to which generally aplasticiser is added. As before the polyaniline can be protonated bymeans of an organic protonation agent and the compatibility substancecan consist of an aromatic compound which, during the manufacturing ofthe material, dissolves the conductive polyaniline and forms a strongmolecular combination therewith, and on the other hand ensurescompatibility between the polyaniline and the insulating polymer.

Although the methods in solution give good results with regard to thepercolation threshold, it is always of great advantage to reduce thisthreshold in order to obtain materials exhibiting a high electronicconductivity containing less conductive polymer (polyaniline) and havingthereby better mechanical and optical properties.

This is because, in the case of conductive composite materialscontaining a polyaniline, the lowering of the percolation threshold ishighly advantageous for the following reasons:

1) Because of the high extinction coefficients of the polyaniline forblue and red light, highly transparent green films can be obtained onlyprovided that very low polyaniline contents are used.

2) The mechanical properties of the insulating polymer host matrix canbe preserved only with a low polyaniline content in the compositematerial.

DISCLOSURE OF THE INVENTION

The object of the present invention is precisely compositions for themanufacture of a conductive composite material from solutions, whichmake it possible to obtain high conductivities with lesser quantities ofconductive polymer.

According to the invention, the composition consists of a solution in asolvent of the following constituents:

a) a conductive polyaniline protonated by means of a protonation agentable to promote the dissolution of the polyaniline in the solvent,

b) an insulating polymer, and

c) a plasticiser for the insulating polymer.

In this composition, the presence of a plasticiser for the insulatingpolymer unexpectedly makes it possible to lower the percolationthreshold of the composite material and to obtain high conductivities.Thus, in this material, the plasticiser not only gives flexibility tothe insulating polymer, but in addition prevents the formation ofaggregates of polyaniline by weakening the adhesion forces between thepolyaniline grains. This results in a better dispersion of thepolyaniline in the insulated polymer host matrix and promotes theformation of a continuous lattice of conductive polyaniline in thecomposite. This makes it possible, as will be seen later, to lower thepercolation threshold of the composite material by a factor of 10, thisbeing for example greater than 0.04 in the absence of a plasticiser andbecoming equal to approximately 0.004 with the plasticiser.

In the composition of the invention, the insulating polymers likely tobe used are polymers generally manufactured in the plasticised statesuch as polyvinyl chlorides and cellulosic polymers.

Advantageously, a cellulose derivative such as cellulose acetate will beused as an insulating polymer.

The plasticisers used are chosen from amongst the normal plasticisersfor these types of polymer. It is possible to use in particular, alkyland/or aryl phthalates, alkyl and/or aryl phosphates and mixtures ofthese compounds.

Advantageously, a mixture of dimethyl phthalate, diethyl phthalate andtriphenyl phosphate is used as a plasticiser.

The conductive polyanilines used in the invention are of theemeraldine-salt form. They can be substituted or non-substituted.

It is also possible to use substituted polyanilines such as thosedescribed in the documents EP-A-0643 397 and U.S. Pat. No. 0,532,631.

In the invention, a polyaniline is used, protonated by means of aprotonation agent able to promote the dissolution of the polyaniline inthe solvent used. Protonation agents of this type comprise an acidfunction and hydrocarbon chains conferring a surfactant character onthem and making them compatible with the generally used organicsolvents, which thereby assists the dissolution of the polyaniline inthe solvent.

By way of example of suitable protonation agents, it is possible tocite: aliphatic and/or aromatic monoesters and diesters of phosphoricacid, for example the alkyl and/or aryl esters of phosphoric acid,arylsulphonic acids and arylphosphonic acids.

In the case of esters of phosphoric acid, the aliphatic monoesters anddiesters are preferred.

Preferably, the protonation agent is chosen from the group consisting ofcamphosulphonic acids, phenylphosphonic acid, dibutyl phosphate anddioctyl phosphate.

In the composition of the invention, the organic solvent can also be ofdifferent types but generally solvents of the phenyl type are preferred,such as cresols, in particular meta-cresol.

In the composition of the invention, the concentrations of theconstituents a) protonated polyaniline, b) insulating polymer and c)plasticiser are chosen so that it is possible to obtain, by evaporationof the solvent, a composite material having a proportion by volume ofpolyaniline greater than the percolation threshold. Generally the ratiosof the concentrations by weight of the solvent, insulated polymer andplasticiser are situated in the following ranges:

cellulose acetate/m-cresol: 5 to 12% by weight, and

plasticiser/cellulose acetate: 30 to 60% by weight.

The compositions of the invention can be used for manufacturingcomposite materials, notably in the form of highly transparentconductive flexible films, by pouring the solution, followed byevaporation of the solvent.

Thus another object of the invention is a method of manufacturing aconductive composite material containing a polyaniline, which comprisesthe following steps:

1) preparing a composition consisting of a solution in a solvent ofconstituents a), b) and c) having the aforementioned characteristics,

2) forming the conductive composite material from the said compositionby evaporation of the solvent.

Generally, the composition is prepared by mixing a first solution of theprotonated polyaniline in the solvent with a second solution in the samesolvent of the insulating polymer and plasticiser.

The invention also concerns an electrically conductive compositematerial obtained by this method, which comprises a cellulose acetatematrix in which there are distributed a protonated conductivepolyaniline and a plasticiser consisting of a mixture of dimethylphthalate, diethyl phthalate and triphenyl phosphate, the materialhaving an electronic conductivity of 10⁻⁶ to 10 S/cm.

Advantageously, the polyaniline content of this material is 0.3% to 5%by weight.

Composition of the mixture after evaporation of the solvent:

polyaniline (calculated according to the base polyaniline) 0.3 to 5% byweight;

protonation agent 0.3 to 7% by weight;

cellulose acetate 60 to 70% by weight;

plasticiser 15 to 40%.

The polyaniline is preferably protonated by means of phenylphosphonicacid.

Other characteristics and advantages of the invention will emerge moreclearly from a reading of the following examples given of course forpurposes of illustration and non-limitatively, with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 4 are graphs illustrating the conductivity of conductivecomposite materials, obtained by the method of the invention, as afunction of the polyaniline content; FIGS. 1 to 4 correspond to the useof various protonation agents.

DETAILED DISCLOSURE OF THE EMBODIMENTS EXAMPLE 1

In this example, a composite material according to the invention isprepared, using cellulose acetate as an insulating polymer, emeraldineprotonated by means of acid dioctyl phosphate as the polyaniline and amixture of dimethyl phthalate, diethyl phthalate and triphenyl phosphateas the plasticiser.

a) Preparation of the protonated emeraldine solution Emeraldine preparedaccording to the method described by McDiarmid et al in L. Alcacer ed.Conducting polymers, Special Applications, Reidle, 1987, pp. 105-119.This polyaniline has the following characteristics: M_(n)=21500 andM_(w)=71000 g/mol as determined by gel permeation chromotography. Theprotonation of this polyaniline is effected by introducing 500 mg ofpolyemeraldine and 891 mg of acid diisooctyl phosphate in 100 g ofm-cresol. The protonation reaction is effected for a week at roomtemperature whilst stirring vigorously. After one week, the soluble andinsoluble fractions of protonated polyaniline are separated bycentrifugation at 5000 rev/min for 15 minutes. Gravimetric analysisshows that 68% by weight of the initial emeraldine has been solubilisedin the meta-cresol by protonation whilst 32% by weight remainsinsoluble.

b) Preparation of the solution of cellulose acetate and plasticiser inm-cresol

For a total weight of 100 g of solution, 10 g of cellulose acetate(Aldrich, molecular weight approximately 50,000 g/mol), 2.5 g ofdimethyl phthalate (99% Aldrich), 2.5 g of diethyl phthalate (99%Aldrich) and 0.2 g of triphenyl phosphate (99% Aldrich) are dissolved in84.8 g of m-cresol, at room temperature.

c) Preparation of conductive composite material

Two grams of the solution of cellulose acetate and plasticiser are mixedin m-cresol, which contains in total 304 mg of cellulose acetate andplasticiser with 1.818 g of the solution of protonated polyaniline inm-cresol (soluble polyaniline fraction separated at a), which contains6.19 mg of emeraldine (estimated as non-protonated emeraldine).

Films are cast from this mixture by slow evaporation of the m-cresol at50-60° C. The films have an emeraldine content of 2% by weight(estimated as non-protonated emeraldine).

The conductivity of the films thus obtained, measured by the standardtechnique using four spikes, is 7.10⁻² S/cm.

Comparative Example 1

The same operating method is followed as in Example 1 for preparing acomposite material from the same solutions, except that no plasticiseris introduced into the cellulose acetate solution.

The conductivity of the film obtained under these conditions is lessthan 10⁻¹⁰ S/cm.

This demonstrates clearly that the use of plasticiser significantlylowers the percolation threshold.

EXAMPLE 2

The same operating method as in Example 1 is followed for preparing thesolution of protonated polyaniline in m-cresol and the solution ofcellulose acetate and plasticiser in m-cresol, but 2 g of the solutionof cellulose acetate and plasticiser containing 304 mg of celluloseacetate and plasticiser are mixed with 0.1658 g of the polyanilinesolution, that is to say 2.09 mg of emeraldine (estimated innon-protonated form). The films obtained from this composition have anemeraldine content of 0.7% by weight (estimation in non-protonatedform). The conductivity of the film measured as before is 3.10⁻³ S/cm.

Comparative Example 2

The same operating method is followed as in Example 2, except that thecellulose acetate solution does not contain a plasticiser. In this way afilm is obtained having a conductivity less than 10⁻¹⁰ S/cm, whichconfirms the results obtained in Example 1 on the beneficial effect ofthe plasticiser.

EXAMPLE 3

In this example, the same operating method is followed as in Example 1but, in order to prepare a film of composite material from the samesolutions, but using camphosulphonic acid as a protonation agent andmixture ratios corresponding to polyaniline contents of the materialranging from 1 to 8% by weight.

FIG. 1 illustrates the results obtained, that is to say the conductivityof the composite material (log σ) according to the polyaniline content(% by weight).

EXAMPLE 4

The same operating method is followed as in Example 1, butphenylphosphonic acid is used as a protonation agent and the solutionsare mixed so as to have polyaniline contents in the material of 0.5% to1.8% by weight.

FIG. 2 depicts the conductivity of the material obtained (log σ)according to its polyaniline content (% by weight).

EXAMPLE 5

In this example, the same operating method is followed as in Example 1,but di-n-butyl phosphate is used as a protonation agent and the twosolutions are mixed so as to have polyaniline content ranging from 0.5to 11% by weight. The conductivity of the material obtained (log σ) as afunction of its polyaniline content (% by weight) is given in FIG. 3.

EXAMPLE 6

The same operating method is followed as in Example 1, but using otherproportions of a mixture of the two solutions in order to vary thepolyaniline content of the material from 0.7 to 4% by weight.

FIG. 4 illustrates the conductivity of the material (log σ) according toits polyaniline content.

The percolation thresholds calculated from the results in FIGS. 1 to 4and of the equation:

σ(f)=c(f−f_(c))^(t)

given previously are as follows:

f_(c)=0.0084 for FIG. 1 (polyaniline protonated by means ofcamphosulphonic acid)

f_(c)=0.0044 for FIG. 3 (polyaniline protonated by means of di-n-butylphosphate)

f_(c)=0.0041 for FIG. 4 (polyaniline protonated by means of diisooctylphosphate), and

f_(c)=0.0005 for FIG. 2 (polyaniline protonated by means ofphenylphosphonic acid).

In the cases of composite materials produced under the same conditionsas those in Examples 3 to 6, but without the addition of theplasticising mixture, the percolation thresholds are ten times greater,for example f_(c)>0.04 in this case.

In addition, microscopic observation of the materials obtained withoutplasticiser shows the presence of aggregates of polyaniline grainswhilst such aggregates do not appear in the case of the materialsprepared with the plasticising mixture.

Thus the electrical conductivity measurements and the microscopicobservations confirm the role of the plasticiser in the lowering of thepercolation threshold.

Another very interesting property of the films of composite materialobtained in the above example is that they preserve the excellentflexibility of the plasticised cellulose acetate.

What is claimed is:
 1. A composition for manufacturing a conductivecomposite material, characterized in that it is formed by a solution ina solvent of the following constituents: (a) a conductive polyanilineprotonated by means of a protonation agent able to promote thedissolution of the polyaniline in the solvent, (b) an insulating polymerselected from the group consisting of a cellulosic polymer and apolyvinylchloride, and (c) a plasticizer for the insulating polymer. 2.A composition according to claim 1, wherein the insulating polymercomprises cellulose acetate.
 3. A composition according to claim 1,wherein the plasticizer comprises at least one compound selected fromthe group consisting of an alkyl and/or aryl phthalate and an alkyland/or aryl phosphate.
 4. A composition according to claim 1, whereinthe plasticizer comprises a mixture of dimethyl phthalate, diethylphthalate and triphenyl phosphate.
 5. A composition according to claim1, wherein the protonation agent is selected from the group consistingof an aliphatic and/or an aromatic monoester and a diester of phosphoricacid, an arylsulphonic acid and an arylphosphonic acid.
 6. A compositionaccording to claim 5, wherein the protonation agent is selected from agroup consisting of camphosulphonic acid, phenylphosphonic acid, dibutylphosphate and dioctyl phosphate.
 7. An electrically conductive compositematerial comprising a matrix of cellulose acetate in which there aredistributed a protonated conductive polyaniline and a plasticizer formedby a mixture of dimethyl phthalate, diethyl phthalate and triphenylphosphate, having an electronic conductivity of 10⁻⁶ to 10 S/cm.
 8. Acomposite material according to claim 7, wherein its polyaniline contentis 0.3 to 5% by weight.
 9. A composite material according to claim 7,wherein the polyaniline is protonated by means of phenylphosphonic acid.10. A composition for manufacturing a conductive composite material,characterized in that it is formed by a solution in a solvent of thefollowing constituents: (a) a conductive polyaniline protonated by meansof a protonation agent able to promote the dissolution of thepolyaniline in the solvent, (b) an insulating polymer selected from thegroup consisting of a cellulosic polymer and a polyvinylchloride; (c) aplasticizer for the insulating polymer; and (d) wherein the solventcomprises m-cresol.
 11. A composition according to claim 10, wherein theratios of the concentrations by weight of m-cresol, the insulatingpolymer and the plasticizer are in the following ranges: celluloseacetate/m-cresol: 5 to 12% by weight, and plasticizer/cellulose acetate:30 to 60% by weight.
 12. A composition according to claim 10, whereinthe insulating polymer comprises cellulose acetate.
 13. A compositionaccording to claim 10, wherein the plasticizer comprises at least onecompound selected from the group consisting of an alkyl and/or arylphthalate and an alkyl and/or aryl phosphate.
 14. A compositionaccording to claim 13, wherein the plasticizer comprises a mixture ofdimethylphthalate and triphenyl phosphate.
 15. A composition accordingto claim 10, wherein the protonation agent is selected from the groupconsisting of an aliphatic and/or aromatic monoester and a diester ofphosphoric acid, an arylsulphonic acid and an arylphosphoric acid.
 16. Acomposition according to claim 15, wherein the protonation agent isselected from a group consisting of camphosulphonic acid, phenylphosphonic acid, dibutyl phosphate and diactyl phosphate.
 17. A methodof manufacturing a conductive composite material containing apolyaniline, comprising the following steps: 1) preparing a compositioncharacterized in that it is formed by a solution in a solvent of thefollowing constituents: (a) a conductive polyaniline protonated by meansof a protonation agent able to promote the dissolution of thepolyaniline in the solvent (b) an insulating polymer selected from thegroup consisting of a cellulosic polymer and a polyvinylchloride, and(c) a plasticizer for the insulating polymer 2) forming the conductivecomposite material from the said composition by evaporation of thesolvent.
 18. A method according to claim 17, wherein the composition isprepared by mixing a first solution of protonated polyaniline in thesolvent with a second solution in the same solvent of the insulatingpolymer and the plasticizer.